Photosymbiotic ascidians inhabiting subtropical waters tend to have gonads in spring and summer, whereas those in tropical waters are usually sexually mature year-round. We studied the seasonality of sexual reproduction in two populations of the photosymbiotic ascidian Didemnum molle (Herdman, 1886), sampling monthly for 12 months. Although the two populations were located only about 20 km apart, their color morphs were exclusively distributed: colonies of one population were always dark gray; those of the other population were mostly brown. The seasonality of sexual reproduction differed greatly between the populations (and thus between the color morphs). Sexual reproduction was limited to summer in the population with dark gray colonies, whereas the population with brown colonies possessed embryos with tails almost yearround. Moreover, the resident crustacean fauna in the colonies also differed between the populations. The microenvironment in each habitat may have caused these differences, but there may also be some physiological differences between the color morphs that affect the seasonality of sexual reproduction and the resident crustacean fauna.